1. (Field of the Invention)
The present invention relates to a focus detecting system capable of detecting a focusing condition of a photo-taking lens.
2. (Description of the Prior Art)
U.S. Pat. No. 4,636,624 issued in Jan. 13, 1987 discloses such a focus detecting system as shown in FIG. 4 of the accompanying drawings. Referring to FIG. 4, the prior art focus detecting system shown therein includes a condenser lens 3 positioned rearwardly of a photo-taking lens 2 on an optical axis 1, a pair of image forming lenses 4a and 4b positioned rearwardly of the condenser lens 3 and arranged in a symmetrical relationship with each other relative to the optical axis 1, and a pair of line sensors (arrays of photoelectric transducer elements) 5a and 5b arranged on an image forming plane of the image forming lenses 4a and 4b. Reference numeral 6 represents a plane equivalent to a surface of a film to be exposed, and reference numeral 7 represents a correlation detector electrically connected with the line sensors 5a and 5b.
A standard light flux a and a reference light flux b passing respectively through different areas 2a and 2b of an exit pupil of the photo-taking lens 2 are sensed by the associated line sensors 5a and 5b which convert patterns of distribution of light into respective electric signals, said electric signals being in turn applied to the correlation detector 7 to determine the correlation in position between the images formed on the associated line sensors 5a and 5b for carrying out an automatic focus detection. The correlation detector 7 subsequently generates a defocus signal which is utilized to move the photo-taking lens 2 back and forth by means of a drive mechanism for the purpose of an automatic focus adjustment.
Where the focus detecting system of the above described construction is incorporated in a single reflex camera, a main mirror 8 upwardly inclined at an angle of 45.degree. and an auxiliary mirror (a reflecting member) inclined downwardly at an angle of 45.degree. are provided on the optical axis 1 between the photo-taking lens 2 and the film equivalent surface 6 so that a photo-taking light flux having passed through the photo-taking lens 2 can be reflected upwardly by the main mirror 8 so as to travel towards a finder optical system and, on the other hand, a focus detecting light flux having passed through a semitransparent mirror area of the main mirror 8 can be reflected downwards by the auxiliary mirror 9 so as to reach the line sensors 5a and 5b.
And, during the actual photo-taking, both of the main mirror 8 and the auxiliary mirror 9 are pivoted upwardly so as to retract out of the optical axis to permit the photo-taking light flux having passed through the photo-taking lens 2 to impinge upon a surface of the film so as to form thereon an image of a target object to be photographed, thereby giving an exposure to the film.
On the other hand, since the line sensors 5a and 5b are merely oriented horizontally relative to the photo-taking lens 2 at a position containing the optical axis 1, it has been found that the automatic focus detection of, for example, a horizontal line is impossible. In order to obviate this problem, the inventor notices an improved focus detecting system so designed as to accomplish the focus detection not only in the horizontal direction, but also in the vertical direction and as to enable the system to perform the automatic focus detection of, for example, the horizontal line. The improved focus detecting system is of a construction wherein the line sensors are comprised of a plurality of line sensors having different positions and directions, some of them being arranged in a horizontal position which contains the optical axis, while the remaining ones are arranged in a vertical position which does not contain the optical axis.
Where the line sensors are arranged in the vertical position, the auxiliary mirror 9 must have a substantial length in order for the focus detecting light flux to be effectively reflected. However, since the length between a point of intersection between the optical axis 1 and the auxiliary mirror 9 and an upper portion (on the side of a pivot axis) is limited by the main mirror 8 and cannot, accordingly, be increased. Because of this, the necessity arises that the main mirror 8 has to be moved forwards.
The forward movement of the main mirror 8 results in increase of the thickness of a camera body, thereby posing a problem associated with the compactness of the camera as a whole.